Mapping edaphic soils' conditions to identify conservation targets for pine barren and sandplain ecosystems in New York State

Abstract Small habitat patches can be important reservoirs for biodiversity, capable of hosting unique species that are largely absent from the surrounding landscape. In cases where such patches owe their existence to the presence of particular soil types or hydrologic conditions, local‐scale edaphic variables may be more effective components for models that identify patch location than regional‐scale macroclimatic variables often used in habitat and species distribution models. We modeled the edaphic soil conditions that support pine barren, sandplain, and related ecosystems in New York State with the purpose of identifying potential locations for biodiversity conservation. We quantified soil percent sand and soil depth of 156 known high‐quality remnant pine barren and sandplain ecosystems to calculate threshold soil characteristics. We then mapped all soils in the state that were at least as sandy and deep as the threshold values we calculated. The total area of our map of suitable soil conditions was over 9500 km2, made up of forested (57%), urban (26%), agricultural (13%), and open (4%) land covers. Our analysis nearly doubled the recognized area of barren, shrubland, and grassland habitat on deep, sandy soils in New York State. Extensive forested and even agricultural cover on these soils could also be the subject of restoration to further support the biodiversity of these unique ecosystems. The presence of extensive soils in coastal and interior New York that, with the appropriate disturbance regime, have the potential to host pine barren and sandplain ecosystems offers a new perspective on these ecosystems' distribution in the past—and about how to better align conservation and restoration to preserve the future.


| INTRODUC TI ON
As the scale of the biodiversity crisis becomes clear (IPBES, 2019), calls for large-scale conservation of existing habitat have taken on renewed importance (Nicholson et al., 2019;Wilson, 2016). While much attention has been given to prioritizing large, mostly intact landscapes (Worboys et al., 2010) that avoid the known ecological traps of small or isolated patches (Murcia, 1995;Wilson et al., 2016), relatively small habitat patches are also vitally important for biodiversity conservation . Such small habitat patches may be remnants of once-larger landscapes that have been mostly lost such as old-growth forest (Chapman et al., 2015) or grassland remnants (Stoner & Joern, 2004) and urban parks (Ives et al., 2016), or the product of edaphically unique conditions that were always patchy on the landscape such as serpentine soils (Kruckeberg, 1985), rocky outcrops (Buschke et al., 2020), and pine barrens (Motzkin & Foster, 2002). Because they differ from surrounding habitat, they may be regional or global hotspots of biodiversity, supporting species that are largely absent from the surrounding landscape or, indeed, anywhere else (Hulshof & Spasojevic, 2020;Wintle et al., 2019).
Thus, identifying, and prioritizing, opportunities to conserve small, isolated patches is of profound importance . Habitat and species distribution models are useful tools for integrating climatic, geomorphic, soil, and hydrologic variables into predictions of the distribution of rare ecosystems and species (Store & Jokimäki, 2003;Williams et al., 2009). For ecosystems and species that specialize on particular soil types or hydrological conditions, local-scale edaphic variables may be more effective predictors for patch location than regional-scale macroclimatic variables often used in habitat and species distribution models (Velazco et al., 2017).
For example, Mann et al. (1999) used soil taxonomy, geologic parent material, and rock fragment characteristics to map potential habitat of threatened limestone glades in Kentucky at both local and regional spatial scales. Likewise, Thorne et al. (2011) used maps of serpentine geology and rare species occurrences to map potential reserves in central California. Such methods can aid in identifying small patches of unique conditions that support regionally and globally significant biodiversity reserves.
Pine barrens, sandplains, heathlands, dunes, dwarf pine plains, and related ecosystems (hereafter referred to as pine barren and sandplain ecosystems) in the northeastern United States are an example of ecosystems that would benefit from such habitat modeling ( Figure 1). They are patchily distributed across the landscape, and a variety of subtypes including pitch pine-scrub oak barrens, coastal oak-heath forests, dwarf pine plains, and maritime dunes are recognized as rare at the state and global level (Edinger et al., 2014).
They are home to dozens of rare and threatened species including plants such as wild pink (Silene caroliniana ssp. pensylvanica), upright bindweed (Calystegia spithamaea), and New England blazing star (Liatris scariosa var. novae-angliae); insects such as the frosted elfin butterfly (Callophrys irus) and the federally endangered Karner blue butterfly (Plebejus melissa samuelis); amphibians such as the eastern spadefoot toad (Scaphiopus holbrookii); reptiles such as the eastern hognose snake (Heterodon platirhinos); and birds such as the whip-poor-will (Caprimulgus vociferous), common nighthawk (Chordeiles minor), and the prairie warbler (Dendroica discolor; Albany Pine Bush Commission, 2017; New York Natural Heritage Program, 2019; Wagner et al., 2003). They are restricted to edaphically dry soil with deep layers of sand or gravel (Corbin & Thiet, 2020;Forman, 1979;Motzkin et al., 1996Motzkin et al., , 1999, but they also require frequent fires or other disturbances to prevent succession to closed-canopy forests (Forman & Boerner, 1981;Kurczewski & Boyle, 2000;Milne, 1985;Motzkin et al., 1996). Though extensive habitat management and restoration efforts (Bried et al., 2014;Little, 1979;Pfitsch & Williams, 2009), and even the reintroduction of extirpated species (Holman & Fuller, 2011;United States Fish and Wildlife Service, 2003), are underway, intact pine barrens and sandplains occupy only a fraction of their historical area due to fire suppression and subsequent succession to forest, as well as conversion to agricultural and urban uses (Motzkin & Foster, 2002;Noss et al., 1995).
Identifying patches of pine barren and sandplain ecosystems offers the opportunity to expand conservation of these important reservoirs of biodiversity. In this paper, we used soil geomorphological variables to model the locations of conditions that support pine barren and sandplain ecosystems in New York State (USA). We analyzed the soil characteristics of known remnants of these ecosystems and extrapolated those characteristics to the rest of the state. We also quantified the current land cover of these potential areas to further narrow conservation targets and to gauge the barriers to successfully restoring biodiversity and ecosystem function. The result was a map that nearly doubled the known area of open barren, shrubland, and grassland cover on suitable soils, while also identifying abundant forest, agriculture, and urban land cover on these soils. We argue that our map can be used to identify opportunities to augment existing, conserved pine barren and sandplain ecosystems in previously F I G U R E 1 Pine barren ecosystem at Albany Pine Bush Preserve (NY). Scattered pitch pine trees are visible, with a mixed understory of perennial lupine and other herbaceous vegetation. Open sand is visible in gaps between plants.
overlooked areas for the benefit of the variety of rare and threatened species they support.

| ME THODS
We selected 27 ecosystem types identified by the New York Natural Heritage Program (NYNHP) that occur primarily on deep sandy soils (Table 1). We did not include ecosystems such as dwarf pine ridges or limestone and sandstone pavement barrens that share many characteristics and species with those in Table 1, but whose thin soils limit tree establishment and autogenic succession to hardwood forest.
We mapped 156 known locations of these focus ecosystem types using data from the NYNHP (Edinger et al., 2014;New York Natural Heritage Program, 2021). We used the United States Geological Survey's Gridded Soil Survey Geographic database (gSSURGO; Soil Survey Staff, 2021; 10 m resolution) to characterize the mean percent sand and soil depth (cm) of each of these 156 pine barren and sandplain ecosystem locations. gSSURGO is a field-validated dataset in the form of a series of geospatial polygons derived from a landscape's soil taxonomy. We did not field validate our map's predicted soil characteristics, instead relying on the gSSURGO database's robustness at the scale of our investigation (Soil Survey Staff, 2017).
We characterized the mean percent sand and soil depth (cm) of where Length g is the length (cm) of each horizon, PercentSand g is the percent sand of each horizon (g), and m is the number of horizons in each soil type.
where Area h and Area i are the areas of each soil type, PercentSand h is the mean percent sand of each soil type, calculated in Equation 1, SoilDepth i is the depth to the nearest restrictive layer of each soil type, and n is the number of soil types in each location.
We established threshold values for sand content and depth that would accurately represent the typical soil characteristics of the focus ecosystems by randomly selecting 109 of the 156 locations (=70%) and calculating the area-weighted mean for percent sand and soil depth (Equations 4 and 5).
where Area j and Area k are the areas of each of the 109 randomly selected location, PercentSand j is the mean percent sand of each location, calculated in Equation 2, and SoilDepth k is the depth to the mean distance to nearest restrictive layer of each location, calculated in The area-weighted mean (± area-weighted SD) percent sand content of the subset of these locations that we used to train our model was 87 ± 11%; the area-weighted mean depth (± areaweighted SD) to a restrictive layer was 193 ± 33 cm ( Figure 2). We used the area-weighted means for percent sand and depth extended to include one area-weighted SD below the mean-at least 76% sand and at least 160 cm depth-as thresholds to define soils most likely to support pine barren and sandplain ecosystems. We applied them to the statewide gSSURGO dataset to create a map of New York's soils where mean percent sand (Equation 1) and depth to nearest restrictive layer were higher than the threshold values. We omitted areas whose land cover was wetlands or open water. The final result was a map of areas in New York where soils are suitably sandy and deep to support pine barren and sandplain ecosystems.
We validated that our modeled locations of deep sandy soils accurately represented conditions that favor pine barren and sandplain ecosystems, and their associated biota, in three ways (Appendix A).
First, we calculated the proportion of the 47 focus ecosystem locations that were not used to generate threshold values (i.e., the remaining 30% of the 156 NYNHP ecosystem locations) that fell within our map of the state's deep sandy soils (Appendix A Table   A1). Second, we tested whether our model avoided conditions that support ecosystems outside our focus ecosystem types by calculating the proportions of areas of the other 147 other native ecosystem types mapped by NYNHP that occurred within our map (Appendix A   Table A1). Finally, we assessed the ability of our model to characterize the location of rare plants and animals that occupy pine barren and sandplain ecosystems using location maps for 58 moths and butterflies, one toad, and five plants that have close affinity to the focus ecosystems (Appendix A Table A2). Most of these species are classified as rare or species of conservation concern at the federal or state level. Sighting dates for plants and animals, as well as the dates of most recent observations of the community data, ranged from 1978 to 2017 (New York Natural Heritage Program, 2019). Most locations were identified as spatial coordinates, though some coordinates were estimated from location names (e.g., a park where the species was sighted) using GoogleEarth coordinates. We calculated the proportion of the known location of each species that intersected with our map of deep sandy soils.
In order to understand the current conditions of the soils our model identified, we intersected our map with a map of United We performed all spatial analysis using ArcMap (10.8.1, ESRI) and data summaries using R (R Core Team, 2022).

| RE SULTS
The known area of the focus ecosystems, namely those that occur primarily on well-drained, sandy soils, identified by the New York   Table A2). For the 29 moths and butterflies whose affinity to the focal ecosystems is high, the overlap was 87%. These species include the federally endangered Karner blue butterfly (98%), the state threatened frosted elfin butterfly (91%), the state species of special concern coastal barrens buckmoth (Hemileuca maia ssp. 5) (87%), and a variety of other species of high conservation concern (Appendix A Table A2).
The occurrences of the one vertebrate for which there was data, the eastern spadefoot toad, was also well described by the soils (79%).  America is in New Jersey's pinelands, but similar ecosystems can also be found on Cape Cod and other coastal beaches and barrier islands of the Atlantic coast (Corbin & Thiet, 2020;Forman, 1979;Foster & Motzkin, 2003). Widely scattered, inland sand deposits from glacial lakes also support pine barren and sandplain ecosystems in Connecticut, Maine, New Hampshire, Vermont (Corbin & Thiet, 2020;Motzkin et al., 1996) and the upper Midwestern US (Radeloff et al., 1999). Modeling of deep sandy soils as potential open-canopy habitat in these other regions has the potential to suggest further opportunities to augment current protected area for the benefit of biodiversity.
Other ecosystems besides those that occur on deep, sandy soils are likely predictable from soil conditions for the purposes of identifying potential conservation and restoration targets (Velazco et al., 2017). Pine barrens and open grasslands in New York and elsewhere in the region also occur on the edaphically thin soils of rocky slopes, summits, and limestone and sandstone plains (New York Natural Heritage Program, 2021). Such globally and regionally rare communities as dwarf pine ridges, sandstone pavement barrens, alvar grasslands and woodlands, and calcareous red cedar barrens host a similar suite of unique plants and animals as the communities that we have previously detailed. Opportunities exist to extend the modeling of edaphic conditions to identify conservation targets for the ecosystems that are restricted to these thin soils (e.g. Manitoba Alvar Initiative, 2012). Serpentine and limestone glade ecosystems, are also restricted to narrow, edaphic, soil conditions (Belcher et al., 1992;DeSelm, 1986;Kruckeberg, 1985;Proctor & Woodell, 1975), each product of specialized geology that creates unique chemical or physical soil conditions. In such cases, soil classification and soil survey data that identify the geologic conditions that drive ecosystem occurrence may be sufficient to build accurate models for potential habitat (Mann et al., 1999;Thorne et al., 2011).

| Implications for the natural history of New York's pine barrens and sandplains
Our analysis does not reveal the distribution of pine barrens and sandplains in times past, but reconstructions from historical maps and aerial photographs in Long Island and Massachusetts reveal extensive pine barren and sandplain ecosystems in the years following European settlement (Foster & Motzkin, 2003;Jordan et al., 2003;Motzkin et al., 1996Motzkin et al., , 1999. Motzkin et al. (1999), for example, found that pinelands existed in over one-quarter of the outwash sand deposits in Massachusetts' Connecticut River Valley. Thus, it is likely that, in the past, the area of pine barrens and sandplain ecosystems in New York State was significantly larger than the several hundred square kilometers they occupy today. Widespread fire suppression and the abandonment of Colonial-era agricultural practices in the 19th and 20th centuries likely initiated succession to closed-canopy forest throughout the region (Foster & Motzkin, 2003;Motzkin et al., 1999;Radeloff et al., 2000). Such forests, which now occupy a majority of deep, sandy soils in New York, are often unable to support the unique and rare species that are characteristic of pine barren and sandplain ecosystems.
Vegetation types on these soils can be quite dynamic over decadal time periods (Foster & Motzkin, 2003;Motzkin et al., 1996Motzkin et al., , 1999. For example, Motzkin et al. (1996) found wide variation in plant cover over time-from grasslands to shrub heath to sparsecanopy pinelands to hardwood forest and back-that shifted dramatically from pre-Colonial times to present. Viewed from this perspective, pine barren and sandplain ecosystems likely coexisted with forests within a dynamic mosaic (sensu Fuhlendorf & Engle, 2004;Wu & Loucks, 1995)  cession, but collectively could support a continuous metapopulation. In this way, the distribution and population dynamics of pine barren and sandplain endemics could have resembled those of serpentine endemics, whose populations are supported by a network of connected patches that form dynamic metapopulations (Harrison, 2011;Harrison et al., 1988;Kruckeberg, 1985). writing -review and editing (supporting).
Howard, D. Rodbell, R. Thiet, and S. Young for help and feedback at various points of the project. The text was improved by comments from two anonymous reviewers. ELF was supported by a Summer Research Fellowship from Union College.

CO N FLI C T O F I NTE R E S T
The authors declare no conflicts of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
A GeoTIFF file of the soils we displayed in Figures 3 and 4 (Table A1).
We were also interested in whether our model avoided conditions that support ecosystems outside our focus ecosystem types, such as mesic forests. We did this by calculating the proportions of areas of the 147 other native ecosystem types mapped by NYNHP that occurred within our map. Our model avoided matches with a variety of ecosystems that are not known to occur on deep sandy soils (Table A1). For example, only 2% of hemlock-northern hardwood and floodplain forests, and less than 1% of the area of Appalachian oak-hickory, beech-maple mesic, and chestnut oak forests occurred within our modeled area (Table A2).

TA B L E A 1
The area of all rare or high-quality native ecosystems as recorded by the New York Natural Heritage Program (NYNHP) data (New York Natural Heritage Program, 2021) and the percentage of that area that occurs on soils identified by our soil model.